A PREFERENCE OF HISTONE H1 FOR METHYLATED DNA

We have identified a clear preference of histone H1 for CpG-methylated DNA, irrespective of DNA sequence. The conditions under which this pr eference is observed allow cooperative binding of H1; the H1-DNA compl exes formed were shown earlier to be 'tramlines' of two DNA duplexes b ridged by an array of H1 molecules, and multiples of these. The prefer ence for methylated DNA is clear in sedimentation assays, which also s how that the preference is greater with increased methylation level, a nd in gel retardation assays with am oligonucleotide containing a sing le methyl-CpG pair; it is shared by the globular domain which also bin ds cooperatively to DNA. A small intrinsic preference of H1 for methyl ated DNA is also apparent in Southwestern assays where the immobilized H1 presumably cannot bind cooperatively. Methylated DNA in H1-DNA com plexes was partially protected (relative to unmethylated DNA) against digestion by MspI but not by enzymes whose cutting sites were not meth ylated, consistent with a direct interaction of H1 with methylated nuc leotides; this was also true of GH1-DNA complexes. H1 variants (spH1 a nd H5) from transcriptionally repressed nuclei have a stronger prefere nce than H1 for methylated DNA, suggesting that this may be relevant t o the stabilization of chromatin higher order structure and transcript ional repression.